1. Field of the Invention
The present invention relates to an image processing device and an image processing method which are suitable for a process of a cone beam CT (computerized tomography) image.
2. Related Background Art
FIG. 2 shows the outline of a cone beam X-ray CT device (or scanner).
In FIG. 2, an X-ray irradiated from an X-ray tube 103 is absorbed and attenuated inside the body of a subject 102, and the X-ray passed the subject 102 is detected on a surface sensor 101. Then, the X-ray tube 103 and the surface sensor 101 are rotated around the subject 102 without changing the relative physical relationship between the X-ray tube 103 and the surface sensor 101, whereby the projection image data of the subject 102 as much as one rotation is acquired. Thus, the projection image data acquired like this is subjected to a reconstruction process, whereby the tomographic image of the subject 102 is acquired. Incidentally, to acquire the same tomographic image, the subject 102 may be rotated as much as one rotation instead of the rotation of the X-ray tube 103 and the surface sensor 101.
In the reconstruction process, a convolution process is first executed to the projection image data as shown in FIG. 3, and the convolution-processed projection image data is then subjected to back projection to each pixel of the reconstruction image as shown in FIG. 4.
To execute the reconstruction process at high speed, for example, a multiprocessor is used. Here, it should be noted that the method of achieving the high-speed reconstruction process by using the multiprocessor is described in “Reconstruction of 3-D X-ray Computerized Tomography Images Using a Distributed Memory Multiprocessor System”, TOHRU SASAKI and YASUSHI FUKUDA, Information Processing Society of Japan Transaction, Vol. 38, No. 9 (hereinafter called the document 1). In this method, the parallel processes are achieved by using the plural processors, and also the data is transferred at high speed. That is, the high-speed reconstruction process is achieved mainly by hardware.
However, the following important problems occur in the cone beam CT. That is, in the conventional technique, the projection image data acquired all over the projection angles is the line data corresponding to the one-dimensional fan beam.
However, in the cone beam CT, the projection image data is the two-dimensional image data, whereby an amount of data necessary in the process is huge. For this reason, if the convolution process is executed wholly to the huge amount of data as in the method described in the document 1, it is inefficient. Thus, it is impossible to achieve the high-speed reconstruction process by the hardware. Moreover, even if the parallel processes are executed, a huge amount of projection image data must be stored in the local memory of each of the processing units, whereby the capacity of each local memory must be made with high cost.